1. Field of the Invention
The present invention relates to a fuel pump control apparatus for controlling an operation to drive a fuel pump for supplying fuel to a fuel injection nozzle of an engine. To put it in detail, the present invention relates to a fuel pump control apparatus that is capable of changing the pressure of fuel supplied to a fuel injection nozzle of an engine with a good response characteristic with respect to a requested fuel pressure in each operating state of the engine and, hence, capable of supplying fuel to a fuel injection nozzle at an amount and a pressure appropriate for the operating state.
2. Description of the Prior Art
In general, the conventional control of a fuel pump for supplying fuel to a fuel injection nozzle of an engine is implemented as a so-called PWM control or a so-called duty ratio control for turning on and off a current input to the pump due to the fact that the PWM or duty ratio control is easy to execute and allows the amount of consumed electric power to be reduced. In addition, in order to obtain information on the operating state of the engine, a variety of sensors are installed at some locations on the engine. Control of an operation carried out by the fuel pump control apparatus to drive the fuel pump is based on signals output by the sensors to supply fuel to a fuel injection nozzle at a fuel pressure required by the engine. When the fuel pressure required by the engine changes due to a variation in engine operating state or a variation in environment temperature, the fuel pump control apparatus makes an adjustment in accordance with such a change to just produce a new fuel pressure required by the engine.
In relation to what is described above, a variety of technologies for changing the pressure of fuel supplied to a fuel injection nozzle have been disclosed. Examples of such technologies are disclosed in Japanese Patent Laid-open No. Hei 8-232790 and Japanese Patent Laid-open No. Hei 7-166993. The following is a brief description of technologies of this type. To begin with, the configuration of the fuel supplying apparatus is shown in FIG. 7 in a simple and plain manner. As shown in the figure, a fuel pump 30 is attached to a fuel tank 32. The fuel pump 30 is linked to fuel injection nozzles 6 installed on an intake pipe 4 by a fuel supplying pipe 34. Fuel in the fuel tank 32 is sucked by the fuel pump 30 and transmitted to the fuel injection nozzles 6 by way of the fuel supplying pipe 34 and a fuel rail 34a by applying a pressure to the fuel.
In addition, a return path 38 is provided between the fuel pump 30 and the fuel rail 34a. Excess fuel is returned to the fuel tank 32 by way of the return path 38. Furthermore, a pressure regulator 36 and a valve 35 are provided on the return path 38. A throttle 33 is installed in parallel to the valve 35. The pressure regulator 36 and the valve 35 are used for adjusting and changing the pressure of fuel supplied to the fuel injection nozzles 6. A fuel pressure sensor 3 for measuring the pressure of the supplied fuel is provided at a proper location in the fuel supply system. The fuel pressure sensor 3 supplies information on the fuel pressure to an engine control unit (ECU) 20.
The fuel pressure in the fuel supply system is controlled by the engine control unit 20 by using a control algorithm based on signals C supplied by a variety of sensors installed at some locations on the engine. The ECU 20 outputs control signals used in the control. Examples of the control signals are a control signal C1 output to the fuel injection nozzles 6 for injection of fuel by opening and closing the fuel injection nozzles 6, a control signal C2 for opening and closing the valve 35 and a control signal C3 for controlling the operation to drive the fuel pump 30. FIG. 8 is a diagram showing a typical waveform of the control signal C1. During a period Ta of the control signal C1, fuel is injected to the engine. It should be noted that the period Ta, that is, the injection timing, is determined almost entirely by the rotational speed of the engine.
On the other hand, FIG. 9 is a diagram showing a typical waveform of the control signal c3. The average voltage of the control signal C3 is (Tb/To) X VB where Tb is the pulse width, To is a period of typically 50 msec or shorter and VB is the voltage of a power supply which has a fixed value of about 12 V. The output of the fuel pump 30 is all but proportional to the average voltage. Thus, since To and VB are constant, the average voltage, that is, the output of the fuel pump 30, can be adjusted by changing Tb. As a result, by setting Tb at a proper value, excessive fuel can be prevented from being supplied to the fuel injection nozzles 6. That is, fuel of an amount appropriate for the operating state of the engine 2 can be supplied to the fuel injection nozzles 6. For example, when the flow rate of fuel returned through the return path 38 is high, the length of the period Tb of the pump control signal C3 is reduced to decrease the flow rate.
The pressure of the supplied fuel is adjusted by opening and closing the valve 35. To put it in detail, when the valve 35 is opened, an attempt is made to change the pressure Pf of the supplied fuel toward a value equal to a pressure Pr adjusted by the pressure regulator 36. When the valve 35 is closed, on the other hand, an attempt is made to change the pressure Pf of the supplied fuel toward a value higher than the pressure Pr adjusted by the pressure regulator 36.
As disclosed in Japanese Utility Model Laid-open No. Hei 3-63764, there is also known a technology whereby a plurality of pressure regulators having different adjusted pressures are arranged in the fuel supply system and the pressure of supplied fuel is adjusted by properly selecting some of the pressure regulators.
However, the technologies for changing and adjusting the pressure of supplied fuel described above have the following problems. In the first place, in the technology disclosed in Japanese Patent Laid-open No. Hei 8-232790, the pressure of fuel supplied to the fuel injection nozzles 6 is changed by opening and closing the valve 35. That is, first of all, the valve 35 is opened or closed in order to change the fuel pressure. Then, the operation to drive the fuel pump 30 is controlled. As a result, it takes time to get the fuel pressure actually changed, giving rise to a problem of responsiveness. In particular, in order to set the fuel pressure at a high value, the valve 35 is closed and then the fuel pump 30 is controlled at a fixed fuel discharging power. Thus, it takes time to get the fuel pressure actually increased after the request for a change in fuel pressure is received. As described above, the control to change the fuel pressure disclosed in Japanese Patent Laid-open No. Hei 8-232790 has responsiveness and stability problems.
As for the technology disclosed in Japanese Utility model Laid-open No. Hei 3-63764, a plurality of pressure regulators are used, raising not only a cost problem but also a problem of difficulty to accommodate the fuel supply system with such an enlarged size in a limited engine room or a limited fuel tank. There is also raised a problem that a big additional restriction is imposed on the pipe design of the fuel supply system. In addition, in order to increase the fuel pressure to a high value, there is also needed a fuel pump that is capable of discharging fuel at a pressure higher than a set pressure of a pressure regulator set for a high pressure. A fuel pump with a large pumping capacity or a large fuel discharging power gives rise to a problem that the size of the fuel pump is enlarged and the amount of electric power consumed thereby is also increased as well. These problems are explained by referring to FIG. 10. FIG. 10 is a diagram showing graphs representing characteristics of the fuel pump. The graphs show relations between a voltage applied to the fuel pump and the discharged flow rate with the fuel pressure taken as a parameter. The graph indicated by notation Pf=Pr.sub.1 is a line representing a relation for a fixed fuel pressure set by the pressure regulator on the low pressure side. On the other hand, the graph indicated by notation Pf=Pr.sub.2 is a line representing a relation for a fixed fuel pressure set by the pressure regulator on the high pressure side. The graphs indicate that, in order to increase the fuel pressure Pf, it is necessary to increase the voltage applied to the fuel pump and, in order to increase the discharged flow rate at a fixed fuel pressure, it is also necessary to increase the voltage applied to the fuel pump as well.
Pay attention to a case in which the fuel pressure is increased to a high value Pr.sub.2. In this case, there is a difference between the fuel flow rate Q demanded by the engine and the fuel flow rate q demanded by a system disclosed in Japanese Utility Model Laid-open No. Hei 3-63764. This is because the fuel flow rate q includes an excess fuel flow rate set by a pressure regulator on a low pressure side and an excess fuel flow rate set by a pressure regulator on a high pressure side. As such, since it is necessary to supply fuel of an amount larger than the fuel flow rate Q demanded by the engine, a fuel pump with a large pumping capacity or a large fuel discharging power is required, giving rise to a problem that the size of the fuel pump is enlarged and the amount of electric power consumed thereby is also increased as well.
The operation to drive the fuel pump 30 is controlled by reducing the flow rate of returned fuel to adjust the amount of fuel supplied to the fuel injection nozzles 6, but the amount of the returned fuel can not be made zero. That is, fuel of at least a fixed amount has to be returned to the fuel tank 32. Once transmitted to the high-temperature engine 2, the returned fuel has a temperature higher than that of the fuel in the fuel tank 32, raising a problem that the fuel in the fuel tank 32 evaporates easily because its temperature increases. Thus, the problem described earlier is not solved completely. There is also proposed another solution called an in-tank return structure wherein the pressure regulator 36 is installed in the fuel tank 32 so that returned fuel passes through only the fuel tank. However, this solution causes problems such as one that the amount of agitated fuel in the fuel tank 32 and the driving amount of the fuel pump 30 can not each be reduced to a minimum.